Applied Thermal Engineering 29 (2009) 2373–2378

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate /apthermeng

Improvement of micro-combustion stability through electrical heating

Junhu Zhou, Yang Wang, Weijuan Yang *, Jianzhong Liu, Zhihua Wang, Kefa CenState Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, Zhejiang, China

a r t i c l e i n f o

Article history:Received 8 April 2008Accepted 4 December 2008Available online 24 December 2008

Keywords:Micro combustorsMicro flameFlame stabilityHydrogenElectrical heating

1359-4311/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.applthermaleng.2008.12.005

* Corresponding author.E-mail address: yangwj@zju.edu.cn (W. Yang).

a b s t r a c t

This experiment investigates the performance of a micro combustor under different operation conditions,and its improvement with electrical heating inside. According to the results, extinction and blow outoccurred at 0.08 and 0.4 L/min, respectively. Electrical heating enhanced the stability effectively. Withelectrical heating power of 1.05 and 4.70 W, the equivalence ratio ranges of stable combustion areextended from 0.362 – 6.52 to 0.178 – 7.66 and 0.126 – 9.43 on average, respectively, and it is more effec-tive in the lean and low-flow-rate cases. The micro flame shows obvious shift downstream at high flowrates. The flames in the lean cases have higher robustness, their reaction region are closer to the nozzleoutlet around 1.17 mm on average than in the rich cases. The heat loss and heat conduction of the microcombustor surface are found to be related to its performance. The rich cases at moderate flow rates have0.086 W lower heat loss on average than the lean cases, which enhances the stability effectively. How-ever, the rich cases have 0.013 W lower heat conduction on average at moderate flow rates, thus moresevere shift downstream occurs.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Micro power generators are based on micro-electromechanicalsystems technology. They can supply electricity for portable elec-tronic devices, such as laptops, cellular phones, which requirehigh-density power source for long periods of time of operation[1,2]. Because electricity is converted directly from hydrocarbonfuels, the micro power generators have energy per unit mass 100times higher than lithium-ion batteries. They show better perfor-mance than conventional batteries if only the efficiency is higherthan 1% [3–7].

Epstein of Massachusetts Institute of Technology suggested thefirst micro power generator [8]. It was actually a micro gas turbinegenerator with volume less than 1 cm3, and was expected to have aelectrical power output of 10–50 W, with hydrogen consumptionrate no more than 10 g/h. However, micro integral combustion en-gines may be impractical, due to the high heat and friction losses.There are also other kinds of micro power generators, such as ther-moelectric, piezoelectric or pyroelectric devices [1,9,10], whichhave the benefits of having no moving parts. The micro combustorscould also provide heat for endothermic reactions, such as steamreforming and ammonia decomposition, which is very importantfor the production of hydrogen for fuel cell [11]. However, the sta-bility of micro combustors is weak. There have been researches toincrease the efficiency and stability of micro combustors.

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Many characteristics of micro combustors, which are ignored inconventional ones, are emphasized, such as: high quenching dis-tance, low diffusion of mass and heat, radical quenching, uniformi-zation of temperature difference and flow laminarization [12].Especially, the heat loss for micro combustors is a key factor totheir stability. Because the surface to volume ratio of micro com-bustors increases dramatically, and their sensible heat generationis proportional to the volume, but the heat loss is proportional tothe surface area [13].

Lots of new methods have been suggested to improve the per-formance of micro combustors, such as ‘‘swiss-roll” combustors[14], which are based on the mechanism of heat recirculation[15]. The combustors increase total reactant enthalpy of theincoming cold reactants, through transferring thermal energy fromthe combustion products to the reactants. Upstream heat transferthrough conduction on the wall to preheat the incoming reactantsplays an important part in stabilizing combustion in tube-shapecombustors. Churchill [16] developed thermally stabilized burnersaccording to this mechanism, but the range of flow rates allowingstable work was limited. Another means is employing catalyticreaction [14]. The lower reaction temperature induces lower heatloss than homogeneous combustion. Radical quenching could beeliminated by inactiving the surface of micro combustors chemi-cally, the process involves high-temperature annealing, surfacecleaning and hydrogen peroxide [17].

Micro combustion has been numerically investigated [18,19].Results of computational fluid dynamics (CFD) simulation provedthe importance of overall heat management in stabilizing microcombustion. The thermal conductivity of the wall has both positive

Nomenclature

V total flow rate (L/min)Ta average temperature (�C)Sw area of wall surface (m2)

Sc area of cross section (m2)e radiative emissivitydb black body radiation constant (W/(m2 K4))Qcov natural convective heat loss rate (W)h heat transfer coefficient (W/(m2 K))Qrad radiate heat loss rate (W)x axial displacement (mm)Qa available chemical energy (W)/lean / near stability limits in lean cases

/ equivalence ratio ((mH2/mAir)/(mH2/mAir)stoi)Tw wall temperature (�C)Tp peak temperature (�C)Qcon average heat conduction rate (W)k thermal conductivity (W/(m K))T1 ambient temperature (�C)Qgen combustion heat release rate (W)Qe electrical heating rate (W)Qloss heat loss rate (W)DT temperature difference (K)/rich / near stability limits in rich cases

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and negative effects [20]. Higher conductivity helps preheat thefeed [1], but increases the detrimental heat loss and causes extinc-tion. The lower one does the opposite [21]. The countercurrentheat-recirculating combustor was also modeled. The resultsemphasized the importance of the heat transfer, and the operationlimits of combustors were quite sensitive to the axial conduction ofwall [22].

In this experiment, a micro straight-tube combustor is tested.We installed an electrical-heating wire in the micro combustor toimprove its stability. The performance of the combustor under dif-ferent flow rates and equivalence ratio is investigated. The heatloss and heat conduction on the wall are analyzed.

2. Experimental apparatus

Fig. 1 shows the schematic diagram of the experimental system,and Fig. 2 shows the details inside the micro combustor. The sys-tem consists of gases feed system, measuring instruments, heatingsystem and a micro combustor.

Two high-pressure tanks with pressure reducing valves sup-plied hydrogen and air. The gases were premixed and then jettedinto the micro combustor through a nozzle with 0.4 mm diameter.

Fig. 1. Schematic of the experimental apparatus. The solid lines are the gas routes,the long-dashed lines are the signal routes, the small-dashed lines are the electricityroutes. Arrows mark the gas-flow direction. The small picture gives the photographof the working micro combustor by the infrared camera.

The nozzle was inserted into the micro combustor with a depth of22 mm. The nozzle outlet is recognized as the axial displacement of0 in the latter discussion. Two thermal mass flow controllers (D07-7B), whose capacities were 2 and 1 SLM (standard L/min), respec-tively, controlled the flow rates of mixture and hydrogen. The massflow controllers and a voltmeter were connected to the digital dataacquisition system (WSP-D806). An infrared camera (ThermaCamS65) measured the temperature distribution on the micro combus-tor surface. The electrical-heating wire was installed in the com-bustor. The wire is made of Chrome Stainless Steel and has theresistance of 0.512 X. The two ends of the heating wire protrudingthe micro combustor were connected to the electricity-control de-vice (SWP-D805), which provided high and low voltage for ignitionand heating, respectively. The micro combustor was a straightquartz tube with 2.06 mm inner diameter, 6.06 mm outer diame-ter, and 82 mm length. To calibrate its surface emissivity, the com-bustor was heated to certain temperature by the heating wire.Then the surface temperature was measured with both a K-typethermocouple and the infrared camera. The emissivity was ob-tained through matching the two results, which was approxi-mately 0.86.

The surface temperatures (Tw) and the stability limits of thecombustor were measured under different operation conditions.All the operation conditions with Tw measured are listed inTable 1. The equivalence ratios of lean and rich mixture (/lean, /rich)have difference to the stability limits less than 0.03 and 0.125,respectively.

The stability limits at different flow rates were found followingthe process: the combustion was started first at / of 1. The flowrate of hydrogen was then adjusted with the total flow rate (V)fixed, until extinction or blow out occurred. Because hydrogen

Fig. 2. The detail schematic of the micro combustor. The temperature distributionon the outer edge of the combustor surface was measured, and the importantlocations for Eqs. (1) and (2) are marked on the x axis.

Table 1The operation conditions with Tw measured. The lean, stoichiometric and richmixtures are tested.

V [L/min] / Qe[W]

Lean Stoichiometric Rich

0.08 0.595 1 6.68 00.12 0.340 1 7.82 00.16 0.527 1 5.08 00.2 0.371 1 9.52 00.24 0.366 1 8.00 00.28 0.351 1 7.56 00.32 0.389 1 7.14 00.36 0.297 1 4.88 00.4 0.387 1 5.90 00.08 0.595 1.050.12 1 1.050.2 0.371 1 9.52 1.050.4 1 1.050.2 1 9.52 4.700.4 0.172 9.49

Fig. 3. (a) Stability limits map for H2/air pre-mixture, with Qe of 0, 1.05, 4.70 W. (b)Available chemical energy vs. flow rates at stability limits with Qe of 0, 1.05, 4.70 W.

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flame was invisible, the infrared camera assisted to detect extinc-tion or blow out.

Heat loss rate (Qloss) and average heat conduction rate (Qcon) ofthe micro combustor are calculated with Eqs. (1) and (2). The for-mer one is integrated from the location of peak temperature (xp) tothe nozzle outlet (xn), which is the reaction area. The latter one iscalculated from the location of peak temperature to the upstreamend of combustor (xend), which is the preheating area. Details areshown in Fig. 2

Q loss ¼ Q cov þ Q rad ¼Z xp

xn

½DT � hþ db � e � ðT4w � T4

1Þ� � dSw ð1Þ

Q con ¼R xp

xendðSc � k � dTw

dx Þdx

xp � xendð2Þ

Qloss is the heat loss rate; Qcov is the natural convective heat lossrate; Qrad is the radiative heat loss rate; DT is the temperature dif-ference between wall and ambient; Sw is the external-wall area; h isthe convective heat transfer coefficient; db is the black body radia-tion constant; e is the radiative emissivity; Tw is the wall tempera-ture; T1 is the ambient temperature; Qcon is the heat conductionrate; Sc is the cross-section area; k is the thermal conductivity ofwall; x is the axial location on the combustor wall, the subscriptsp, n and end are peak temperature, nozzle outlet and combustor up-stream end, respectively.

The errors in calculating heat loss and heat conduction are ana-lyzed. The independent parameters measured in the experimentsare: wall temperature, ambient temperature, combustor dimen-sion. A constant value is taken as the emissivity of the micro com-bustor, its error is lower than 4.6%. Uncertainty of empiricalcorrelation of h is less than 5% [23]. Error of Tw is 2%, accordingto the instruction of infrared camera. Errors of x and combustordiameter are both less than 1%. Error of k is 8.4%. Conclusively,the maximum possible uncertainty of Qloss and Qcon are 12.7%and 8.7%, respectively.

3. Results and discussion

3.1. Flammability limits

Fig. 3a shows the stability limits of the micro flame. Both blowout and extinction were observed. Extinction occurred easily at0.08 L/min, where the flame quenched without shift downstream.Blow out occurred at 0.4 L/min instead, where the flame shifteddownstream and resided at the combustor outlet. However, shiftdownstream accompanied with extinction occurred at moderate

flow rates. The profiles of corresponding available chemical energy(Qa) at stability limits are given in Fig. 3b, which are more intui-tionistic. Qa is the energy generated through complete combustion.

At 0.08 L/min without electrical heating, the upper stability lim-it was at / of 3.76 and the lower one at 0.420. The stability of leancase prevailed, with lower Qa (2.51 W vs. 2.75 W). Then the flam-mable region expanded sharply from 0.12 to 0.28 L/min. The richcases improved better, with / increasing from 5.64 to 8.91. Thelean cases had relatively lower stability. Because they had higherthermal capacity (1.26 kJ/m3K) than the rich ones (1.17 kJ/m3K),thus required more heat to reach ignition temperature. At 0.4 L/min, blow out occurred. On one hand, in the narrow tube, wherethe expansion of flame was severely constrained, the practicalburning velocity could be lower than the theoretical one. Whenthe flow velocity exceeded the burning velocity, blow out occurred.On the other hand, there was no space for the circumfluence sec-tion. The flame relied on the heat recirculation along the wall forcontinuous combustion. While insufficient heat was recirculatedat high flow rates, blow out occurred.

With Qe of 1.05 W, the stability improved. In the rich cases, /increased from 3.76 to 7.64 at 0.08 L/min. In the lean cases, theelectrical heating improved the stability dramatically (close to 0and beyond the measurable range of the flowmeter). At the flowrates lower than 0.28 L/min, the maximum H2 concentration inthe rich cases was lower than 81%, but the minimum one in thelean cases was at least 7%. Thus electrical heating was more effec-tive to lean mixture at low flow rates. It is probably because thatthe lower thermal diffusion of lean mixture helped preserve heat

Fig. 5. Wall temperature vs axial displacement under stoichiometric conditions,with electrical heating power of 1.05 and 4.70 W.

Fig. 6. The temperature distributions along the wall at /lean and /rich, withoutelectrical heating.

2376 J. Zhou et al. / Applied Thermal Engineering 29 (2009) 2373–2378

in the reaction region, thus the higher peak temperature improvedthe stability better.

Higher Qe improved the stability better. At 0.2 L/min, / in-creased from 0.347–7.04 to 0.125–10.7, after heated with 4.70 W.At 0.08 L/min, the lean one was extended from 0.420 to almost 0.But at 0.4 L/min, it was ineffective to prevent blow out. Becausethe electrical heating was not powerful enough to decrease thepreheating and reaction time.

3.2. Temperature distribution

Fig. 4 shows the representative temperature profiles under stoi-chiometric condition, without electrical heating. The locations ofthe peak temperature correspond to the flame locations. The pro-files of the temperature distribution are not symmetrical to theirpeak temperatures (Tp). The first half with sharper decline worksas a preheating and ignition source for the incoming cold gas. At0.2 L/min, Tp reached 502 �C, and was located at 3.44 mm. But xp

shifted upstream at 0.120 L/min. Because at the lower flow rates,it required a shorter distance for preheating. However, xp did notshift downstream with flow rates increasing. At 0.4 L/min, theflame retreated to 1.90 mm. It is noticed that Tp reached 658 �C.The high flame temperature caused high flame propagating veloc-ity, thus the flame retreated.

Fig. 5 shows the temperature distribution with electrical heat-ing. While heated with 1.05 W, Tp at 0.12, 0.2, 0.4 L/min increasedto 450, 498, 667 �C, respectively, and xp retreated obviously (from3.44 to 1.94 mm at 0.2 L/min). At 0.2 L/min with 4.70 W, xp re-treated by 3.35 mm compared to the non-heating case, and Tp in-creased to 547 �C. Conclusively, electrical heating increased thetemperature and reaction rate, thus the flame retreated due tothe lower reaction time.

The temperature distributions at /lean and /rich are shown inFigs. 6 and 7. At 0.2 L/min, the heating and non-heating cases arecompared. In the lean case, xp shifted downstream to 5.61 mm.After heated with 1.05 W, it retreated to the nozzle outlet. There-fore, electrical heating helps inhibit shift downstream in the leancases. At /rich of 9.52, xp stayed almost the same (5.72 mm), beforeand after heated with 1.05 W. However, with Qe of 4.70 W, xp re-treated to nozzle outlet. Therefore, inhibiting shift downstreamin the rich cases requires more energy. It is consistent with the sta-bility limits at 0.2 L/min aforementioned in Fig. 3, that heating wasmore effective to improve the stability in lean cases. At 0.08 L/min,there was obvious temperature increase after heated, indicating Qe

compensated the heat lost and increased the reaction temperature,

Fig. 4. Wall temperature vs axial displacement under stoichiometric conditions,without electrical heating.

Fig. 7. The temperature distributions along the wall at /lean and /rich, with Qe of1.05 and 4.70 W.

thus inhibited extinction. At 0.4 L/min, Qe was adjusted to 9.49 W,to sustain the reaction with extremely low / of 0.172. xp had severeshift downstream, indicating that the mixture experienced longdistance of preheating.

Fig. 8. Peak temperature location vs. flow rate at /lean and /rich, without electricalheating.

Fig. 10. Average heat conduction along the axis from the flame center to theupstream end of micro combustor, at /lean and /rich, without electrical heating.

J. Zhou et al. / Applied Thermal Engineering 29 (2009) 2373–2378 2377

Fig. 8 shows the location of peak temperature vs. flow rate at/lean and /rich. From 0.08 to 0.36 L/min, xp increased in direct ratioof flow rate, indicating a steady process of shift downstream. At0.4 L/min, the flame was difficult to sustained, slight shift down-stream would result blow out, thus it reached stability limits whilethe flame still located at the nozzle outlet. It is noticed that the dif-ference of xp between /lean and /rich became larger and larger asthe flow rate increased. It indicates the rich cases required moretime for reaction and preheating at higher flow rates.

3.3. Energy distribution

Qloss and Qcon at different flow rates are given by Figs. 9 and 10.From 0.16 to 0.4 L/min in lean cases, Qloss increased form 0.955 to1.67 W. At 0.4 L/min, xp decreased, indicating shorter reaction area,thus less heat were lost to the environment. However, Qloss in the richcases was 0.086 W lower on average than the lean ones from 0.16 to0.36 L/min. Correspondingly, the rich ones had higher stability(Fig. 3). It is attribute to the high thermal diffusion of rich mixture,which inhibited the exist of hot area.

Heat conduction makes the main part of heat recirculation [24],because it works as a bridge for transporting energy from the flamezone to preheating zone. In the lean cases, Qcon increased from 0.127to 0.242 W at the flow rates from 0.08 to 0.40 L/min, because theincreasing amount of reactants required more heat for preheating.Qcon in rich cases was lower, thus more severe shift downstream oc-

Fig. 9. The heat loss vs flow rate at /lean and /rich, without electrical heating.

curred (Fig. 8). At 0.4 L/min, where blow out occurred, the rich mix-ture required much higher Qcon to stabilize the combustion.

Electrical heating had little effect to inhibit blow out. At 0.4 L/min, after the power was adjusted to 9.49 W, the lower limit wasextended to 0.172. However, 2.82 W heat was lost (1.15 W higherthan the case with Qe = 0), and Qcon decreased to 0.0857 W (notshown in the figures), indicating the flame was not self-sustainedand the heating wire took its place as an ignition source. (The wiretemperature was measured to be 466 �C, higher than the ignitiontemperature).

The thermal properties of combustor wall have twofold effects toflame stability. On one hand, low thermal conductivity decreasesthe heat recirculation, and induces blow out. On the other hand,high thermal conductivity increases heat loss, and induces extinc-tion. Therefore, composite material with exterior ceramic and inte-rior metal is suggest, because it could increase the axial heatconduction and decrease the radial heat loss simultaneously.

4. Conclusion

The micro combustor was tested. Extinction occurred at the lowflow rate (0.08 L/min) due to heat loss, and blow out occurred atthe high flow rate (0.4 L/min) due to the imbalance between burn-ing velocity and gas velocity and insufficient heat recirculation.With electrical heating, the extinction was inhibited. The stabilitylimits of / below 0.36 L/min were broadened from 0.363–6.56 to0.104–9.77 on average, after heated with 4.70 W. However, electri-cal heating was less effective to inhibit blow out at 0.4 L/min. Afterincrease the electrical power to extremely high value, the heatingwire became the ignition source, thus the micro flame was notself-sustained.

The rich cases had 0.086 W lower heat loss on average than thelean ones, which induced higher stability. The lean cases had high-er heat conduction along the combustor wall by 0.013 W on aver-age. Thus the shift downstream is less severe than in the rich cases.The locations of peak temperature never crossed 10 mm from thenozzle outlet in the lean cases. Moreover, with electrical heating,the flammability limits in the lean cases were extended dramati-cally (from 0.420 to almost 0 in the maximum). It is attributed tothe lower thermal diffusion of lean mixture.

Acknowledgements

This work is supported by the National Nature Science Founda-tion of China (No. 50606030), Research Fund for the Doctoral

2378 J. Zhou et al. / Applied Thermal Engineering 29 (2009) 2373–2378

Program of Higher Education of China (No. 20060335124), Programof Introducing Talents of Discipline to University (No. B08026) andNational Science Foundation for Distinguished Young Scholars (No.50525620).

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